[In the posting passed away, and don’t ask how, the question of the spelling and misspelling of the word DILEMMA cropped up. The following was the response which also included information on meaning and usage:]

That’s absolutely amazing! My entire life and I have never known – and
clearly have never noticed – any spelling other than DILEMNA. And I would definitely have taken bets, and lost!

Before I go any further, I want to mention that I found it very interesting that in the link 1 you provided, the author gave this link 2, which he said was where he saw the spelling DILEMNA that initiated his discussion. But if you check this link you will notice that the two instances in which the word appears have now been changed to DILEMMA, and it is my guess that the article referred to in link 2 was edited, possibly due to some negative feedback on the spelling.

In any case, I was flabbergasted by the fact that the spelling DILEMNA, which I know and love, appeared in no dictionary that I could find, and no print source I checked even acknowledged that there was an alternate spelling or misspelling issue. However, I did find the following at Wordsmith.org by Anu Garg, July 22, 2001:

Subject: Curious Dilem(?)a

. . . Recently, a group of friends got together to discuss the word ‘dilemma.’ The discussion came up because an editor changed one of our ‘dilemnas’ to ‘dilemma,’ and was incredulous that anyone would be so foolish as to insert an ‘n’ in this word. However, every one of us (and I'm talking about roughly 20 people) remembered being taught to spell the word D-I-L-E-M-N-A, and also remembered being chastized for the stupidity of using two "m's."

Is this one of those mass hysteria events, or is there something here? I distinctly remember a fourth-grade teacher going on and on, in a very loud drone, about the spelling of this word, and when I write the word, I say it to myself with a little emphasis on that pesky "n."

We are all in our 40's and 50's, which might or might not be relevant. Some youngsters thought to harass us about Alzheimer's, but we beat them into submission, and await your expert opinion. Do you have any insights? Anything you can tell us would be greatly appreciated.
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For Anu Garg’s response, which in a nutshell says that he never heard of this spelling, it makes no etymological sense, and it is probably mistakenly based on the pattern of ‘lemn’ as in the word ‘solemn,’ see http://wordsmith.org/awad/awadmail39.html But I really wish the questioner had said where she was from and if it was NYC, I could easily have been the 21st person agreeing that that was how I was taught to spell it.
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In researching this topic, I found plenty of discussion on the definition/usage of DILLEMMA, which you will note below, but except for Anu Garg's above response, nothing on the spelling question:

American Heritage Dictionary (4th edition, 2004)

DILEMMA noun

1) A situation that requires a choice between options that are or seem equally unfavorable or mutually exclusive.

2) Usage Problem. A problem that seems to defy a satisfactory solution.

3) Logic. An argument that presents two alternatives, each of which has the same consequence.

Usage Note: In its main sense dilemma refers to a situation in which a choice must be made between alternative courses of action or argument. Although citational evidence attests to widespread use of the term meaning simply “a problem” or “a predicament” and involving no issue of choice, 74 percent of the Usage Panel rejects the sentence Juvenile drug abuse is the great dilemma of the 1980s. It is sometimes claimed that because the di- in dilemma comes from a Greek prefix meaning “two,” the word should be used only when exactly two choices are involved. Nevertheless, 64 percent of the Usage Panel accepts its use for choices among three or more options.
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And here is a usage opinion which is in pretty good agreement with the above more recent one:

Harper Dictionary of Contemporary Usage (2nd edition, 1985):

Usage Panel Question

DILEMMA in its traditional use refers to a situation in which one is faced with the necessity of choosing one of two equal alternative. Frequently of late it appears in the more generalized sense of “acute problem” but not one necessarily one offering two equal alternative solutions. Thus one reads: “Drug abuse is the major dilemma of our day.” Do you accept this broader sense of dilemma?

Would you use it in your own speech? Yes: 30%. No: 70%.

In writing Yes: 29%. No: 71%.
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Charles Funk had the following to say on the related phrase in his Hog on Ice and Other Curious Expressions:

ON THE HORNS OF A DILEMMA: A dilemma, in logic, is a form of argument in which a participant finds himself in the embarrassing predicament of having to make a choice of either of two premises, both of which are obnoxious; it is a trap set by an astute person to catch an unwary one, like answering yes or no to the question, “Have you stopped beating your wife?” Because one may be caught and impaled upon either of the alternatives, each of them has been called a “horn.” Medieval scholars, writing in Latin, used the expression, argumentum cornutum, horned argument. Nicolas Udall, in his translations of the adages collected (in Latin) by Erasmus explains the saying in the language of 1546: “Thys forked questyon; which the sophisters call an horned question, because that to whether of both partyes a bodye shall make a direct aunswere, he shall renne on the sharpe poynete of the horne.”
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Facts on File Encyclopedia of Word & Phrase Origins had the following:

HORNS OF A DILEMMA:Lemma in Greek, means a thing taken for granted, deriving from lampanien, “to take.” Affixing the Greek di, “two,” we have dilemma, two things take for granted. Therefore, in logic, a dilemma is an argument that forces a person to choose one of two things taken for granted, both of which may be untrue or unacceptable (e.g., “Do you still beat your wife?”). Philosophers in the Middle Ages also called each lemma of a double lemma a horn, because it was so easy to get caught on the sharp point of each coining the Latin expression argumentum cornutum, “a horned argument.” Since both terms meant the same thing, they were eventually wedded in the phrase to be caught on the horns of a dilemma, to be unable to make up one’s mind about which course of action to choose.
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Brewer’s Dictionary of Phrase & Fable:

THE HORNS OF A DILEMMA: A difficult choice in which the alternatives appear equally distasteful or undesirable. Greek lemma means ‘assumption,’ ‘proposition,’ from lambanein, ‘to grasp.’ A dilemma is a double lemma, or what the ‘schoolmen’ called an argumentum cornutum, ‘horned argument.’ The allusion is to a bull that will toss you whichever horn you grasp.
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Oxford English Dictionary along with some of its early quotes:

DILEMMA noun:

1) In Rhetoric. A form of argument involving an adversary in the choice of two (or, loosely, more) alternatives, either of which is (or appears) equally unfavourable to him. (The alternatives are commonly spoken of as the ‘horns’ of the dilemma.) Hence in Logic, a hypothetical syllogism having a conjunctive or ‘conditional’ major premiss and a disjunctive minor (or, one premiss conjunctive and the other disjunctive). [Very different views have been taken by different logicians as to what syllogisms are properly dilemmas; several of the arguments commonly so called being considered by some writers to be only ordinary conjunctive syllogisms, constructive or destructive. See FOWLER, Deductive Logic, v. §4.] [[If you are confused by the various logic definitions, which don’t all seem to be saying the same thing, you are not alone!]]

<1551 “DILEMMA, otherwise . . . called a horned argument, is when the reason consisteth of repugnant members, so that what so ever you graunt, you fall into the snare.”—Logike (1589) by T. Wilson, page 34b>

<1725 “A DILEMMA becomes faulty or ineffectual . . . when it may be retorted with equal force upon him who utters it.”—Logic by Watts, III. ii. §6>

<1837-8 “An hypothetico-disjunctive syllogism is called the DILEMMA or horned syllogism.” Ibid. 352 “If the disjunction . . . has only two members, the syllogism is then called a DILEMMA in the strict and proper signification. If . . . three . . . members, it is called trilemma, etc.”—Logic (1866) by Sir W. Hamilton, I. pages 351 & 352>

<1887 “In disputation, the adversary who is refuted by a dilemma is said to be ‘fixed on the horns of a dilemma’; he is said to rebut the dilemma, if he meet it by another with an opposite conclusion. Ibid. 122 It seems less arbitrary and more systematic to define dilemma as ‘a syllogism of which one premiss is a conjunctive and the other a disjunctive proposition.’”—Deductive Logic by Thomas Fowler, page 121>

2) Hence, in popular use: A choice between two (or, loosely, several) alternatives, which are or appear equally unfavourable; a position of doubt or perplexity, a ‘fix’.

<1590 “Every motion was intangled with a DILEMMA: . . .the loue of Francesco gaue such fierce assaults to the bulwarke of her affection . .. the feare of her Fathers displeasure . . .draue her to meditate thus.”—Greenes Neuer Too Late (1600) by Robert Greene, page 19>

<1655 “He is reduced to this doleful DILEMMA; either voluntarily, by resigning, to depose himself; or violently, by detrusion, to be deposed by others.”—The church-history of Britain by Fuller, IV. i. §53>

<1796 “Kosciusko was. . . reduced to the unpleasant DILEMMA of being obliged either to kill the father or give up the daughter.”—The American Universal Geography by Morse, II. page 297>

<1841-44 “In the DILEMMA of a swimmer among drowning men, who all catch at him.”—Essays, Experience in Works (Bohn). I. page 189>

<1888 “They were . . . in the DILEMMA of either violating the Constitution or losing a golden opportunity.”—The American Commonwealth by James Bryce, II. liii. page 332>

And this is all that the Oxford English Dictionary and the rest had to offer with not a peep on the alternative spelling/misspelling question - perhaps they deemed it too loutish to be worthy of comment! (>:)

So when, how, and where did the spelling DILEMNA come about?

Firstly, the earliest example I found was from 1845 (see quotes below).

Secondly, it is obviously a misspelling because the word derives from the Greek lemma and there is no history, as far as I could find, of any Greek lemmas turning into lemnas.

Thirdly, I did note with some interest that all the 19th-century quotes I found (including the one’s below) seemed to have their origin in publications from the U.S. Northeast (New York, Pennsylvania, and Connecticut), so there is a chance that this is the area in which the errant spelling was born (as was I).

Fourthly, the nonstandard spelling was not just used by doofuses and the ignorati, but by very respectable folks – folks like me! (<:) – in journal articles, newspapers, magazines, etc., all of which I assume had editors to catch this type of thing. Well, it seems, many editors were similarly misinformed.

My best guess as to how this came about (in agreement with Wordsmith above) is that on the model of such words as condemn, column, indemnity, and solemn someone mistakenly substituted an N for an M in a popular 19th-century school spelling book or guide and the misspelling propagated. It’s hard to imagine how, other than with a scenario similar to this, such a spelling, which appeared in no dictionaries, could have so thoroughly infiltrated the system and been so convincing to so many people who normally should have known better. But until a smoking gun is found, no one will be able to say for certain how this came about. In the mean time, I would definitely rate this up there as a great English orthographical mystery.

As a preface to my quotes below, there are some important search pitfalls I would like to mention, which, incidentally, aren’t unique to my search for DILEMNA:

1) There are many hits, which are just misinformation. For example, if one searches for the misspelled ‘prisoner’s dilemna’ (a famous math problem from game theory) by Powers, the first hit is by a bookseller who just plain got the spelling wrong; but it sure does look legitimate sitting there with ISBN number and all for twenty bucks. However, if one checks on Barnes & Noble or Amazon, one quickly finds that this bookseller had it wrong.

2) There are many false positives where the character recognition program made a booboo and it is only by checking the original image that one discovers this. This was the case with a few of the 23 hits I got using Google’s Book Search for “Prisoner’s Dilemna.” Similarly, if one searches for the book Anatomist’s Dilemna, Google Book Search comes up with the spanking new book Anatomist’s Dilemna (2008) by Goldstone. Except for one thing. The book Goldstone wrote was actually Anatomist’s Dilemma with two M’s, which the publisher proceeded to rename The Anatomy of Deception – I guess they couldn’t handle the spelling dilemma! I have tried to avoid such pitfalls in the following selected list of quotes:

<1845 “. . .the friends of the Speaker wished to extricate him from a DILEMNA into which a little too much haste had driven him.”— Adams Sentinel (Gettysburg, Pennsylvania), 24 March, page 4>

<1848 “And this puts him [[President James Polk]] in a DILEMNA: he must retire from this chosen field of his glory without the ill-gotten fruits of his successful military exploits, or he must prosecute his war from this time forward, for the naked purpose of subjugation and dismemberment.”— The American Whig Review, Vol. 7, Issue 1, ‘The President’s Message—The War,’ page 10> [[published in New York]]

<1851 “. . . the great men of the nation of both political parties, set themselves to work for the purpose of rescuing the country from the DILEMNA in which it had been placed . . .”—The proceedings of the Union meeting, held at Brewster's Hall, December 24, 1850, page 31> [[published in Connecticut]]

<1853 “The machine is extricated from this DILEMNA by the inertia of the fly-wheel . . .”—A Dictionary of Science, Literature & Art by W. T. Brande, page 1170> [[this edition was published in New York. Some later and earlier editions were published in London. The London 1867 edition I checked had the correct spelling DILEMMA]]

<1863 “Yet such a DILEMNA in no way clears up the difficulties of this most inexplicable case.”— The Living Age, Vol. 78, Issue 1000, page 234> [[New York]]

<1874 “If you insist on this version of the letter of the law, we shall insist that you be consistent, and accept the other horn of the DILEMNA, An Account of the Proceedings on the Trial of Susan B. Anthony, page 160> [[New York]]

<1894 “We understand that the court considered itself placed in a DILEMNA since any other interpretation would have made § 11, of the Act of 1855, discriminate against Christianity.”—The American Law Register and Review, Vol. 42, No. 7, (First Series), Volume 33 (New Series, Vol. 1), July, page 524> [[University of Pennsylvania]]

<1901 “The difficulty in reconciling the two points of view led one of Schopenhauer’s correspondents to propose a DILEMNA: either the will is a thing-in-itself . . . or . . .”—The Philosophical Review, Vol. 10, No. 5, September, page 566> [[Cornell University, New York]]

<1917 “. . . he added that this ‘places the supporters of subsidence in a DILEMNA.’” —Proceedings of the National Academy of Sciences of the United States of America, Vol. 3, No. 11, November, page 652>

<1937 (book title) “The American Elm: Its Glorious Past, Its Present Dilemma, Its Hope for the Future” by American Forestry Association>

<1944 (book title) Digest of Myrdal's "An American DILEMNA,” and the Report of the President’s Committee on Civil Rights by Samuel S. Wyer>

<1968 (book title) The Terrible Choice: The Abortion DILEMNA by Robert E. Cokke, Based on the proceedings of the International Conference on Abortion sponsored by the Harvard Divinity School and the Joseph P. Kennedy Jr. Foundation.>

<1977 “Congress must wrestle with the DILEMNA of possibly contributing to unemployment—particularly among youth—as it considers raising the minimum wage.”— Chicago Tribune, 5 September, page C8>

<2005 (book review) “[[Charles]] Shapes's DILEMNA is summed up even before he finishes his meal: his is a life ‘spent being nice--and never in the process having to be good.’”—The Review of Contemporary Fiction, 22 June>

<2006 “The experts face a DILEMNA: the further they go to reach safe water, the weaker their patient is becoming.”—The Independent on Sunday (London), 22 January>

<2006 “In many games, two players make their decisions simultaneously, such as in the simplest of games, the Prisoner’s DILEMNA.”— (in faculty research paper) The Last Line of Defense by Wein & Atkinson of Graduate School of Business & Institute for Computational and Mathematical Engineering at Stanford University, 10 October, page 3> [[famous mathematical problem in game theory]]

<2007 “For the Democrats, including the three leading presidential candidates deep into their endgame maneuvers in Iowa and New Hampshire, the classic DILEMNA was posed: Do you want a big issue for the upcoming campaign or will you take a few policy steps forward now and try for the rest later?”—Boston Globe (Massachusetts), 23 December>

Ken Greenwald wrote:
My entire life and I have never known – and clearly have never noticed – any spelling other than DILEMNA. And I would definitely have taken bets, and lost!

You were on the horns of a double duckweed! (Lemna is the genus of duckweeds which are small floating water plants that can take over your pond or aquarium and are very hard to eradicate if you don't want them.)

Not only did you manage to not see dilemMa, you must have avoided (or avoided connecting the terms) lemma in mathematical/logical proofs (I remember using lemmas in 10th grade geometry) and various trilemmas (mostly religious - Epicurus, C.S. Lewis, ...). There's even a tetralemma in eastern philosophy.

Russ, You mean that wasn't Zorn's lemna? I just looked up LEMNA in the OED and, interestingly enough, it appears that the word LEMNA (duckweed) has experienced the reverse problem that DILEMMA has (note the double Ms in 3 out of the 5 quotes they provide).

<1753 “Lemma [[sic.]] . . . is . . . the name of a small water plant well known to the antients [[ancients]]. . . confounded by late writers among the duck weed kinds.”—Cyclopædia - Supplement by E. Chambers>

<1789 “Gold and silver fishes . . . will also feed on the water-plant called lemna (ducks' meat).”—The Natural History and Antiquities of Selborne by G. White, II.liv. page 269>

<1801 “‘This,’ replied Belinda, ‘is what “Th'unlearned, duck~weed; learned, lemma [sic.], call,” and it is to be found in any ditch or standing pool.’”—Belinda by M. Edgeworth, II. xxi. page 298>

<1802 “The Convallarian Vorticella is frequently found on the stalks of the lemma [[sic.]] or duckweed.”—Animal Biography (1813) by Bingley, III. page 490>

<1967 “As a result of its small size and the ease with which it may be cultured . . . Lemna is eminently suitable for laboratory experiments.”—The Biology of Aquatic Vascular Plants by C. D. Sculthorpe, vii. page 204>

Incidentally, pentalemmas and hexalemmas have always made me nervous with an urge to run off a cliff, whereas dillemmas give me a craving for pickles.
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I have always spelt Dilemma using an "n". I am 49 and only found out just then when looking up an "Ethical dilemna" online for an Ethics assignment that it is commonly spelt Dilemma! I have always been a very good speller which is mainly as I have avidly read since I was small and have obviously learned most of my spelling from books. I know that I have always read and spelt dilemna with an n. I think it is amazing that we were all brought up spelling it like that. How simply life would have been if I had have known that it was actually spelt dilemma! The reason being is that every time I have said the word, I would hesitate on its pronunciation as I was always tempted to pronounce it Dil-em-na and would have to stop myself.